Giant Ancient Galactic Cloud Nurseries

The simplest and yet most mind-boggling thing about stargazing is the fact that, wherever you are on the planet, you aren’t actually looking up but rather looking out into the Universe. This is because gravity, the stuff that keeps us firmly planted down on Earth, and the Earth going around the Sun, pulls things inwards from every direction. 

This fundamental force can be intriguing, especially if we consider it in the extreme, and because we know that gravity is related to mass (thank you Einstein) we also know that the more mass we have - the more gravity is present.

At really large scales of mass - like those present in galaxies, or clusters of galaxies, gravity can create a rather quirky yet interesting visual effect, known as a gravitational lens - which allows astronomers to see light (sometimes magnified and amplified) from beyond the large intervening mass.

Now, astrophysicists from Melbourne’s Swinburne University of Technology, North Carolina State University, and the W.M. Keck Observatory in Hawaii have used gravitational lensing and advanced spectroscopic techniques as tools to study the nature of giant nebulae of atomic hydrogen, known as Damped Lyman-α systems (DLAs), which fuel the formation of stars in young galaxies.

Their work focussed on two of these DLAs, which are so distant from Earth that the light they observed had left the region over 11 billion years ago. In fact, without the magnification from gravitational lensing, these distant sources would be too faint to observe in such detail. The astronomers, whose research was published last month in Nature, used the Keck telescope’s innovative Cosmic Web Imager to closely study the spectrum of this light and analyse the chemical composition of these massive gaseous objects.  

Jeff Cooke, a professor at Swinburne and one of the paper’s authors noted that “DLAs are crucial in understanding how galaxies were formed, but have traditionally been extremely difficult to observe.”

“By using the powerful capabilities of the W.M. Keck Observatory, some fortuitous alignments of galaxies, and Einstein’s General Relativity, we are able to observe and study these massively important objects in a completely new way, giving us insight into how the stars and planets around us were formed.”

As DLAs drift through distant regions of space their presence is normally only detected when they pass in between us and bright astronomical sources like active galactic nuclei (AGN) or gamma-ray bursts (GRBs). These massive clouds of hydrogen span tens of thousands of lightyears - the two in this study are about two thirds the size of our Milky Way - so it is possible to use AGN or GRBs as backlights but this method only illuminates the small part of the DLA passing in front of the luminous objects.

So what is gravitational lensing? Lead author Dr Rongmon Bordoloi, from North Carolina State University, explained that “gravitationally lensed galaxies refer to galaxies that appear stretched and brightened”. One of the fundamental things that we learn from Einstein’s general theory of relativity is that massive bodies warp the spacetime around them which, as Dr Bordoloi put it, causes “the light bends as it travels toward us, so we end up looking at an extended version of the object – it’s like using a cosmic telescope that increases magnification and gives us better visualisation.”

Dr Bordoloi’s group observed a particular gravitationally lensing arc in the sky formed by the strong gravitational forces of a galaxy around 20 billion light-years away and further magnified and stretched by a relatively nearby massive galaxy cluster, about 5 billion lightyears away. This cosmic magnifying glass allowed the astronomers to get a closer look at the two DLAs - located in between at comoving distances of roughly 17 and 19 billion light-years from Earth.

"By utilising the latest technology at Keck and a little luck with the alignment of gravitationally lensed galaxies, we have greater insight into the workings of our universe than ever before,” Professor Cooke says.